The present invention is directed to improvements in dampers and the like. More particularly, the present invention is a hybrid hydraulic and elastomeric damper which may find application in a helicopter rotor system, for example.
Bonded elastomeric components have been utilized in helicopter vibration control products for many years. Their relative simplicity, light weight, reliability, energy storage capacity, multi-axis spring rate capability and maintenance-free operation, have popularized their usage in helicopter applications such as lead-lag dampers, pylon isolators, landing gear components, stabilator mounts, drive couplings and avionic isolators. Bonded elastomer components are also used in such non-rotary wing applications as vehicle suspensions and impact absorbers.
More recently, hydraulic dampers have been utilized for certain of the helicopter applications including lead-lag dampers and landing gear components. These systems typically employ conventional hydraulic seals/bearings which wear, over time, leading to leaks. Further, some of the systems employ sophisticated valving which adds cost, is subject to blockage or other failure, is highly frequency dependent and susceptible to accelerated wear caused by intrusion of sand and dust. Lastly, when these dampers fail, it is typically as a result of loss of hydraulic pressure leading to total loss of damping. Such a cataclysmic failure can jeopardize the operation of the aircraft.
While bonded components are widely used and have performed exceptionally well, they are not without their limitations in certain applications. For example, lead-lag dampers are constructed of highly damped elastomer in order to be able to provide all the motion damping needed. However, these highly damped elastomers can produce a wide range of stiffness and damping characteristics as a result of variations in strain levels associated with the input disturbance. Further, highly damped elastomers are limited by the level of fatigue strain to which they may be subjected and their lower stiffnesses and loss factors at high amplitudes can be disadvantageous. "Loss factor" is defined as the ratio of damping stiffness, K", to elastic stiffness, K'. In some applications, the low loss factors at low amplitudes attendant the highly damped elastomers can lead to unacceptable limit cycle oscillations. Lastly, the compromises in such properties as tensile strength, tear strength, shear fatigue and creep can limit the practical loss factor of a damper employing highly damped elastomer to around 0.8.
The present invention overcomes the difficulties of the two types of dampers by combining the best features of the bonded elastomer- and hydraulic-type dampers into a single hybrid damper. A pair of annular elastomeric members are bonded to and interconnect first and second concentric cylindrical elements in such a way as to create first and second fluid-containing chambers. These chambers are interconnected by a narrow annular passageway through which the fluid is throttled as movement between the concentric cylinders deforms the elastomeric walls of the respective chambers. A volume compensator is integrated into the design.
Energy is dissipated by means of three distinct modes:
1) the hysteresis of the elastomer; PA1 2) contraction and expansion of the fluid as it is throttled through the annular gap; and PA1 3) shearing of the fluid between the two concentric cylindrical members. PA1 a) no dynamic seals to wear out or cause leaks; PA1 b) fewer components; PA1 c) no catastrophic loss of damping (gradual, observable change); PA1 d) less frequency-dependence; PA1 e) eliminates need for extremely close manufacturing tolerances; PA1 f) longer, more predictable service life; PA1 g) "fail-safe" system operation since loss of the fluid will not result in total loss of damping (spring restraint and damping of elastomer is still available); PA1 h) sand and grit usually kicked up by operation of a helicopter do not have the adverse effects on performance; PA1 i) can accomodate bending moments across the damper. PA1 a) higher loss factors are available; PA1 b) longer fatigue life; PA1 c) more linear performance; PA1 d) smaller space envelope for equivalent damping or greater damping in an equivalent space; PA1 e) less sensitivity to changes in dynamic characteristics (amplitude); PA1 f) maintains a higher loss factor at low strain levels than highly damped elastomers; PA1 g) less susceptible to creep or to permanent set.
These three damping modes combine to dampen relative axial, torsional and skewing motions between the two components linked by the damper, such as the rotor and the blade of a helicopter, for example. Since a significant portion of the damping is provided by the two fluid modes, the elastomer can be selected for other characteristics, such as strength and shear fatigue, rather than for its high damping.
This hybrid damper, identified as a Fluidlastic.RTM. damper, is superior to a conventional hydraulic damper in the following ways:
The Fluidlastic.RTM. damper has the following advantages over a conventional bonded elastomer damper:
This damper affords a versatile hybrid damper whose fluid and elastomer characteristics can be engineered to provide a wide range of varying design performance criteria enabling it to meet a wide variety of needs.
Various other features advantages and characteristics will become apparent after a reading of the detailed description which follows.